This invention relates generally to an apparatus and method for steering a vehicle, and more specifically to a rack-independent actuator.
Many current steering system designs have replaced the hydraulic power steering pump with electrically assisted systems based on fuel economy, modularity, engine independence, and environmental issues.
With electrically actuated or electrically assisted steering systems there is a significant servo mechanism design challenge associated with the need to maintain proper kinematical constraint, while at the same time, providing reasonable insulation from the drawbacks of tolerance stack up which may produce system lock up.
Although a successful servo mechanism design may appear to be a combination of basic xe2x80x9ccataloguexe2x80x9d mechanisms (e.g. ball-screw, gears, belts, various joints, etc.), the way these are used in combination represents an unmistakably cardinal feature of this art.
The current state of engineering meets these concerns by anticipating the stresses likely to be encountered by designing heavy-duty components. Needless to say, these designs are expensive to manufacture, have excessive performance challenges because of the increased inertia and friction, and add to the overall weight of the vehicle.
The system is powered by a rotary type electric motor. The motor has speed reducers and rotary-to-linear actuators to achieve feasible size and linear actuation. The actuation unit is decoupled from the directionally unwanted loads by providing universal joints (or an equivalent degree of freedoms) at either end. One universal joint is mounted to the housing that holds the motor rotary-to-rotary speed reducer and the movable shaft of the linear-to-rotary actuator, and the other is mounted to a member that is linearly moved by the linear-to-rotary actuator.
The use of universal joints (or gimbals), which provides kinematical degrees of freedom to prevent non-axial loads, also prevents bending moments on the rotary-to-linear actuator. In particular, such loads may result from the misalignment of the shafts and/or non-axial loading from other components. This situation may produce undesirable friction and high stresses resulting in loss of efficiency and/or undesirable steering feel. By avoiding the non-axial loads, the mechanization becomes feasible for all types of linear-to-rotary mechanizations, which today are limited to very special ball-screws.